Glare (also known as specular reflection) produced when light from a bright window or lamp is reflected from the surface of a glossy print can completely mask the image. Although the surface reflects only about 5% of the light (it depends somewhat on the incident angle), the windows or lamps are much brighter than the surfaces they illuminate (typically about 30 times as bright in a business office). So the glare brightness usually is at least equal to the brightness of highlight areas of the prints, and much brighter than dark areas of the prints. The viewer must position or tilt the print to minimize glare, or if the print is held by someone else or is mounted on the wall, the viewer must change position and viewing angle to reduce the glare.
In attempts to make glare less objectionable, some print papers are manufactured with semigloss finishes, in which a microscopically wavy surface scatters the surface reflections to a limited extent. Substantially the same amount of light is reflected as with a glossy print, but the glare appears to the viewer to be less intense since the reflections are directionally scattered. But because of the broader scatter, the glare is harder to remove from view by positioning or tilting the print.
Going even further, print papers are also manufactured with matte finishes in which a microscopically irregular surface scatters the surface reflections to a great extent. In effect, with matte materials the viewer always sees a more or less uniform surface reflection, no matter what the viewing angle is. This uniform surface reflection reduces contrast and significantly grays the portions of the pictures that should be deeply colored or black.
A partial reduction in the light reflected from a picture surface can be made by causing the light to undergo two surface reflections before it reaches the viewer's eye. This approach is used by Whitehead in his "Non-Reflective Graphic Surface Display Device" of U.S. Pat. No. 4,883,341. This device consists of a transparent plate grooved on the top and having its bottom surface in optical contact with the graphic surface being displayed, so that reflections from the graphic surface are eliminated and only reflections from the grooved surface remain.
In Whitehead's preferred embodiment (column 5, line 7 et seq), the grooves have right angle sawtooth profiles, so that glare light reaching the viewer's eyes will have undergone two reflections. At each reflection the light intensity is reduced to about 5% of its intensity before reflection. So after two reflections, the intensity of the glare is only about 5% of what it was with just one reflection (without the grooved display device). This appreciably reduces the glare, but because of the eye's roughly logarithmic response, the glare pattern produced by bright windows or lamps is still visible and distracting.
Another difficulty with Whitehead's preferred embodiment is that the right angle grooves serve as prisms to refract the light coming from the graphic surface. This seriously scrambles the image a viewer sees. For the dimensions Whitehead gives, the graphic surface appears as if it were divided into vertical strips about 1 mm wide, with each odd numbered strip appearing to be displaced about 1 mm to the left, and each even numbered strip appearing to be displaced about 1 mm to the right. This destroys the fine detail in the picture.